1. Field of the Invention
The present invention relates to a process for the production of porous phenolic resin fiber and a porous fibrous structure made of phenolic resin possessing excellent heat-resistance and adiabatic property in addition to high flexibility.
2. Description of the Prior Art
The phenolic resin fibers are generally produced in such manner that a thermoplastic resin called phenolic novolac which has been obtained by condensing at least one phenol compound with an aldehyde compound represented by formaldehyde in the presence of an acidic catalyst is molten in a non-oxidative atmosphere by heating and then subjected to a crosslinking reaction with an aldehyde compound such as formaldehyde under various reaction conditions including the use of a basic or acidic catalyst or the use of a basic catalyst followed by an acidic catalyst (See, for example, Japanese Patent Publn. No. Sho.48-11284).
From the past, phenolic resin fibers are used as a material for various kinds of safety goods in case of emergency, adiabatic materials, packing of sealing materials and a substitute for asbestos, utilizing their good heat-resistance, adiabatic property, and chemicals-resistance based on their molecular structure. As the phenolic resin fibers show a good yield of a product on carbonization and are excellent in physical properties when processed to active carbon fibers, the phenolic resin fibers are useful also as a precursor of carbon fibers or active carbon fibers.
However, phenolic resin fibers as organic fibers are not comparable, even if they possess excellent heat-resisting property, with inorganic fibers such as glass fibers or ceramic fibers in heat-resisting temperature, and so may not be used under severe conditions. Thus, a number of studies have been made from the past for improving various properties of the phenolic resin fibers.
In Japanese Laid-open Patent Appln. No. Sho. 53-94626, there is disclosed under the title "a process for manufacturing flame-resisting fiber or flame-resisting fibrous structure" an economical process for the production of flame-resisting fibers possessing excellent heat-resistance, which is characterized by bringing phenolic resin fibers in a non-oxidative atmosphere to a heat-treatment conducted at 280-400.degree. C. under relaxative conditions for the fibers. In Japanese Patent Publn. No. Sho. 50-34125, there is disclosed under the title "infusible, non-combustible hollow fibers and a process for producing same" a process for the production of infusible, non-combustible hollow fibers which are excellent in bending strength, chemical-resistance and adiabatic property characterized by crosslinking uncured phenolic resin fibers inwardly from the outer peripheral portion thereof up to a depth of 20-90 % of the cross-sectional area thereof and then extracting the uncrosslinked portion of the resin in the central part of the fibers with a solvent.
In case attention is paid particularly to heat-resisting and adiabatic properties of the fibers, however, the phenolic resin fibers obtained in the above mentioned prior art processes are still unsatisfactory in these properties; heat-resistance is certainly improved but adiabatic property is not improved in case of the solid fibers and adiabatic property is certainly improved but heat resistance is not improved in case of hollow fibers, while adiabatic property is improved but the field of industry producing or using frictional materials, adiabatic materials, packing/sealing materials and safety goods, improvement in heat-resistance and adiabatic property of the product is always required to warrant the performance of the product under severe conditions. Under the circumstance, there is a great demand for developing a new process for producing phenolic resin fibers which are remarkably improved in heat-resistance and adiabatic property without damaging their other useful properties such as chemical-resisting property, infusibility and flexibility as fibers.